The invention disclosed herein relates generally to a method and apparatus for processing semiconductor wafers. More particularly, the present invention relates to a method and apparatus for reducing scrap and improving the reliability of integrated circuit devices during cleaning by eliminating the exposure of the semiconductor wafers to light during rinsing and drying.
A series of processing steps, known to those skilled in the art, are required to fabricate integrated circuits on semiconductor wafers. Many of these integrated circuit fabrication steps involve cleaning the semiconductor wafers prior to each new process used to form the integrated circuit devices. It is desirable to ensure that the semiconductor wafers are thoroughly cleaned by removing any remnants of prior processing, particulate contaminants and debris from the wafer surface prior to further processing. For example, after wet chemical operations, such as etching polymers, removing photoresist and removing residue, the semiconductor wafers typically undergo a cleaning process that includes chemical cleaning in a solvent or acid/solvent mixture prior to rinsing the wafers in water and then drying the semiconductor wafers.
Wafer cleaning machines typically utilize high purity wet processing to clean the semiconductor wafers. Conventional techniques used to rinse and dry the semiconductor wafers include overflow or cascade rinsing, dump rinsing, and spin rinsing/drying. In overflow rinsing, the water flows into the bottom of a processing chamber holding the wafers to be rinsed and then the water overflows or cascades into a second chamber which collects the rinse water. Dump rinsing uses the rapid deployment of water from a rinse tank to remove debris from the semiconductor wafers. In dump rinsing the water flows into the top of a processing chamber holding the wafers to be rinsed and then the bottom of the chamber is opened to allow the water to rapidly flow out of the chamber. Another technique for rinsing and drying semiconductor wafers uses a spin rinser/dryer which combines a water rinse spray with spinning the wafers in an inert gas, preferably a gas with high purity, to remove the water using centrifugal force and evaporation. It is common to combine these rinsing techniques so that the semiconductor wafers are often cleaned by overflow rinsing or dump rinsing or both, and then are further cleaned by spin rinsing/drying.
Wafer cleaning machines typically have a processing chamber with a transparent window to allow an operator to observe the rinsing and drying steps. These machines generally clean a batch of wafers at one time. The wafers are typically loaded into carriers or racks that each hold about twenty-five wafers to make handling the semiconductor wafers easier and to increase throughput during cleaning. The carriers holding the wafers are loaded into wafer cleaning machines so that cleaning of the wafers occurs while the wafers are loaded in a carrier.
The water used for rinsing, known to one skilled in the art, typically is purified, deionized water that has all of its solids, minerals and contaminants removed. The water for rinsing also typically contains carbon dioxide (xe2x80x9ccarbonatedxe2x80x9d) to inhibit corrosion of the integrated circuit devices. The high purity inert gas used to dry the semiconductor wafers typically contains only trace amounts of impurities, generally on the order of parts-per-billion or parts-per-trillion of impurity content. The high purity inert gas may be comprised of gases such as nitrogen, argon, helium, carbon dioxide, hydrogen and other inert gases know to those skilled in the art.
Despite the use of carbonated water to inhibit corrosion, the front wafer in each carrier exhibits corrosion where the wafers are rinsed and dried in a conventional wafer cleaning machine having a transparent window. Typically, about one of every twenty-five wafers is scrapped due to excessive corrosion of the front wafer in a carrier of wafers that is cleaned in these machines, particularly during cleaning of wafers having integrated circuit devices with exposed metal. Even if the corrosion is not prevalent enough to immediately reject the integrated circuit devices on the front wafer, the reliability of these integrated circuits is of great concern. Although the integrated circuit devices on the front wafer may initially test as good devices, the partially missing metal lines caused by corrosion create a high current density region within the circuit, decreasing the reliability of these integrated circuits.
There is thus a need for a method and apparatus for reducing scrap and improving the reliability of integrated circuit devices by decreasing the corrosion of semiconductor wafers that are produced during rinsing and drying the wafers in conventional wafer cleaning machines having transparent windows.
It is an object of the present invention to provide a method and apparatus for reducing scrap and improving the reliability of integrated circuit devices during cleaning.
It is another object of the present invention to provide a method and apparatus for eliminating light-induced galvanic corrosion produced during rinsing and drying of semiconductor wafers in wafer cleaning machines having windows.
It is another object of the present invention to make the fabrication of advanced integrated circuit devices of increasingly smaller dimensions feasible.
It is yet another object of the present invention to provide a method and apparatus for achieving these improvements that are cost effective and easily implemented in existing wafer cleaning machines.
The above and other objects are achieved by using a light blocking material to cover the transparent window of the wafer cleaning machine while the semiconductor wafers are being rinsed and dried. Since corrosion is observed on the first wafer in a carrier, and the amount of corrosion was reduced in areas of the first wafer that are covered or shielded by the carrier, the corrosion is determined to be caused, at least in part, by the exposure of the semiconductor wafers to light during rinsing and drying. The ambient light from the room that enters the wafer cleaning machine through its transparent window provides sufficient energy to produce galvanic corrosion of the integrated circuit devices that are exposed to the light during rinsing and/or drying. The front wafer in a carrier is exposed to the greatest amount of ambient light while the other wafers are to a varying degree shielded from the light entering the transparent window of the wafer cleaning machine by the front wafer and the carrier itself. Accordingly, the front wafer in a carrier exhibits more corrosion than the other wafers in the carrier that are cleaned in the same batch since the front wafer is exposed to more light, and the light provides the energy to produce the galvanic corrosion. The light-induced galvanic corrosion is further confirmed by cleaning a carrier of wafers wherein the front wafer does not exhibit corrosion when it is reversed so that its back, and not its integrated circuit devices, is exposed to the ambient light during rinsing and drying.
By applying a light blocking material to the window, ambient room light is not permitted to enter the processing chamber of the wafer cleaning machine eliminating light-induced galvanic corrosion of the semiconductors during rinsing and drying thereby reducing scrap and improving the reliability of integrated circuit devices. The light blocking material consists of a light absorbing or a reflective material, which prevents light from entering the window of the wafer cleaning machine.